Usually coaxial cables must fulfill certain electrical and mechanical conditions for use in high frequency ranges over a wide frequency band, for example, from the MHz to the GHz range. The following electrical properties are desirable:
low signal attenuation PA0 high return loss PA0 load carrying capacity PA0 high flexibility PA0 long service life under flexing and/or drum winding load PA0 sturdy design resistant to strain and/or pressure loads, and PA0 small cable diameters.
In addition, the following mechanical properties are desirable:
A low signal attenuation is desirable in order to transmit signals over distances of maximum length. A high return loss makes wave impedance of the cable as constant as possible over its length. Changes in wave impedance along the cable lead to disturbing signal reflections and signal reflux. For a certain load carrying capacity, the inner and the outer conductors of the cable need a certain minimum diameter for the low frequency range. With rising frequency, the skin effect becomes more and more noticeable. An important role is played by the dielectric between the inner and the outer conductor, in particular by its dielectric constant and its dielectric loss factor.
A cable with an inner conductor in the form of massive copper or a massive copper tube has very good electric properties. However, it does not have the desired mechanical properties. A massive copper tube causes the cable to be virtually unbendable and cannot be wound around a cable drum.
Generally, the goal for a cable is one with optimal compromise between the desired electrical and mechanical properties. Cables with focus on low signal attenuation are known as "Zellflex" or "Flexwell" cables and their inner conductor is in the form of a corrugated copper tube. Its structure resembles a flexible shower tube so as to cause the inner conductor to be flexible to a certain degree. Nevertheless such cables are not very flexible and these cables can hardly be wound up on drums, i.e., their bending radius is very large.
Coaxial cables with an inner conductor in the form of braided flat or round conductors arranged around a plastic core have better bending and drum winding properties. However, they are relatively complex and cost-intensive to manufacture. When subjected to frequent bending and drum cycles, they have a relatively short service life.
There is a need for a low-attenuation coaxial cable which optimizes desired electrical and mechanical properties as well as manufacturing costs.